By default you are not notified of any new topics posted. To ensure you are notified about any new posting you should turn on Notification for that particular board.
To turn on notification there is a button Notify at the top right of the screen when you are in any particular board, just click on this to turn on notification. You must do this separately for each board you are interested in, or you can do it only for topics you are interested in.
You will then be advised of the first item in any new topic if you have selected board notify or any new addition to a topic if you have selected a topic. But only the first change, no further notification will be sent unless you visit the board. This is to avoid you being inundated with notifications.
If you reply to any topic at the bottom of the page where you enter your reply there is a box to tick if you wish to add notification to the topic. You may have to click on the + sign next to Attachments and other options to see this box, is is normally ticked by default unless you have changed your options in your profile.
By turning on notification you will ensure that you do not miss any new topics that are posted.